What is the NHRA engine displacement limit for Top Fuel dragsters?

The NHRA engine displacement limit for Top Fuel dragsters is 500 cubic inches.

That limit describes engine size, not horsepower. Modern Top Fuel engines are supercharged V8s based on the Chrysler Hemi design, with a customary 4.1875-inch bore and 4.5-inch stroke. Despite the relatively compact displacement, enormous fuel flow, nitromethane-based fuel, high boost and specialized tuning let these engines produce several thousand horsepower.

The number is specific to NHRA Top Fuel rules, so it should not be generalized to every drag-racing class or sanctioning body. Pro Stock also uses a 500-cubic-inch NHRA limit, while other categories and organizations can use different formulas. A related point often missed is that Top Fuel cars now commonly race over 1,000 feet rather than the traditional quarter-mile, largely for safety as speeds rose.